Many types of input devices are presently available for performing operations in a computing system, such as buttons or keys, mice, trackballs, joysticks, touch sensor panels, touch screens and the like. Touch screens, in particular, are becoming increasingly popular because of their ease and versatility of operation as well as their declining price. Touch screens can include a touch sensor panel, which can be a clear panel with a touch-sensitive surface, and a display device such as a liquid crystal display (LCD) that can be positioned partially or fully behind the panel so that the touch-sensitive surface can cover at least a portion of the viewable area of the display device. Touch screens can allow a user to perform various functions by touching the touch sensor panel using a finger, stylus or other object at a location dictated by a user interface (UI) being displayed by the display device. In general, touch screens can recognize a touch event and the position of the touch event on the touch sensor panel, and the computing system can then interpret the touch event in accordance with the display appearing at the time of the touch event, and thereafter can perform one or more actions based on the touch event.
Single-sided mutual capacitance touch sensor panels typically include a plurality of sense elements distributed across a substrate. Each sense element is separated from an associated set of drive elements by a distance sufficient to enable the sense element to detect when a stimulating voltage has been applied to a particular drive element. When a finger, stylus, or other conductive element is situated proximate to a particular region of the touch sensor panel, a portion of the charge driven by the stimulating voltage escapes via the conductive pathway formed by the finger, stylus, or other conductive element. The amount of charge coupling detected by the sense element is therefore reduced relative to the amount of charge coupling that would be detected absent the conductive pathway. A touch region can then be calculated based upon determining which sense elements have transmitted reduced signals for a particular sensed period.
In some touch sensor panels, the touch surface is situated in front of a liquid crystal display module (LCM). The LCM generates electromagnetic noise which can adversely affect operation of the touch sensor panel. In order to protect the touch sensor panel from the noise generated by the LCM, several techniques have been utilized. In some devices, for example, a conductive shield is deposited upon the back side of the touch sensor panel. The conductive shield adequately filters out noise generated by the LCM, but it also increases the costs of manufacturing the touch sensor panel because deposition and/or patterning of conductive elements is required for both sides of the touch panel substrate. In other devices, a set of reference elements is added to the touch panel layout such that each sense element includes a proportionally sized reference element. The reference elements are spaced far enough from the drive elements so that the capacitive coupling between the reference elements and the drive elements is small or negligible. Signals detected by the reference elements are subtracted from signals detected from associated sense elements in order to filter out the noise common to both elements. This approach, however, consumes a substantial amount of space on the touch sensor panel and can result in undesirably large pixel sizes or smaller sense elements yielding decreased performance.
Additionally, some touch sensor panels are arranged such that successive sense elements are non-uniformly spaced. This often occurs in layouts where the sense elements are distributed in an asymmetric manner among the drive elements, or where common routing must traverse a portion of the touch sensor panel in order to reach its destination. The non-uniform spacing between sense elements introduces errors in algorithms adapted to determine a touch region from a set of reported sense signals, since these algorithms often assume that the sense elements are evenly spaced across the entire touch surface. In some cases, the non-uniform spacing between sense elements can even create regions of the touch sensor panel that are unresponsive to touch input (i.e., “dead zones”).